1. Field of Invention
The present invention relates to an ink, an ink cartridge and an ink jet recording method.
2. Description of the Related Art
An ink jet recording method is such a recording method that minute droplets of an ink are applied to a recording medium such as plain paper to form an image, and is rapidly spread owing to low price of an apparatus itself and improvement of recording speed. In general, a recorded article obtained by the ink jet recording method is low in fastness properties of an image thereof compared with a silver salt photograph. In particular, when the recorded article is exposed to light, humidity, heat or environmental gas present in air, such as an ozone gas for a long period of time, there is a problem that a coloring material of the recorded article is deteriorated to easily cause change in color tone or fading of the image.
The fading of an image recorded using an ink having a hue of black involves color tone change due to the deterioration of a coloring material in many cases. For such a problem, various proposals have been made in which the use of an ink containing a coloring material having a structure excellent in ozone resistance improves the ozone resistance of an image. For example, a proposal is made on an ink in which the use of a compound having an easily associating molecular structure as a black coloring material enables recording an image improved in ozone resistance (see Japanese Patent Application Laid-Open No. 2005-139427 and International Publication No. WO 2012/014954).
As described in Japanese Patent Application Laid-Open No. 2005-139427 and International Publication No. WO 2012/014954, with respect to a coloring material (dye) having a novel structure, such a structural designing that the oxidation potential is raised by incorporation of an electron attracting group, and the coloring material is made to be hardly oxidized by suppression of electron transfer has been in the recent year's mainstream. However, a coloring material thus designed easily polarizes and easily associates. Therefore, although the ozone resistance of an image is improved, there is a problem in that the color tone change is liable to occur in the case where the associating state changes.
The change in the associating state of a coloring material occurs depending on the polarity of an ink, specifically, the dielectric constant of water, a water-soluble organic solvent and the like in the ink. In the ink jet recording method, it is usual in order to record an image having a desired color tone that a plurality of inks are superposingly applied to a recording medium. However, a water-soluble organic solvent contained in another ink superposingly applied exerts an influence on an easily associating coloring material to resultantly change the color tone, and an image having a desired color tone cannot resultantly be recorded in some cases. The color tone change is more easily visible in a neutral hue, that is, a region where a* and b* in the L*a*b* colorimetric system prescribed by CIE (Commission Internationale de l'Eclairage) are nearly zero. Therefore, the color tone change is an important problem in an ink containing a black coloring material.
Further in an ink containing a coloring material designed in an easily associating structure in order to raise the ozone resistance of an image, liquid components such as water evaporate from an ejection orifice of a recording head to resultantly concentrate the ink; such a peculiar problem that the coloring material is liable to cohere and the sticking resistance decreases resultantly arises.
Therefore, it is an object of the present invention to provide an ink which can record an image excellent in ozone resistance, suppressed in color tone change in an environment where various types of water-soluble organic solvents are present, and having a more uniform color tone, and is excellent in sticking resistance. It is another object of the present invention to provide an ink cartridge and an ink jet recording method using the ink.